1. Field of the Invention
Exemplary aspects of the present invention relate to a semiconductor device, and more particularly, to a semiconductor device including an electrostatic discharge protection (ESD) circuit connected to an internal circuit between an input-output terminal and a ground terminal.
2. Description of the Related Art
As an electrostatic discharge (ESD) protection circuit for protecting an internal circuit of a semiconductor device, a protection element called a GGNMOS (gate grounded negative channel metal oxide semiconductor) transistor, in which a gate, a source, and a substrate are all connected to a GND (ground) potential, is generally used.
In a GGNMOS transistor, when a positive electrostatic surge is applied to a terminal connected to the drain, a substrate potential increases due to an avalanche current generated by an avalanche breakdown of the drain, thereby activating a parasitic NPN bipolar transistor. Due to the activation of the parasitic NPN bipolar transistor, an impedance between the drain and the source rapidly decreases to flow a large current, so that a voltage decreases to a hold voltage Vh, in a so-called snapback phenomenon. As a result, thermal breakdown of the PN junction occurs.
However, a high-voltage GGNMOS transistor is subject to being damaged due to the so-called Kirk effect, in which, when an end of a LOCOS (local oxidation of silicon) film has a high electrical field, lots of electrons are trapped in a defect layer at the edges of the LOCOS film, resulting in local leakage of electrical current and breakdown of the GGNMOS transistor. Therefore, the high voltage GGNMOS transistor cannot function as a protection element.
As an alternative to the GGNMOS transistor, use of a diode as an ESD protection element is possible. However, such an arrangement also has a problem in that, since the diode has a too-large on-resistance in motion, the diode needs a very large layout area to allow a sufficient amount of electrical current to flow in order not to damage the internal circuit.
Compared to the diode, a high-voltage PMOS (positive channel metal oxide semiconductor) transistor, that is, a SGPMOS (source-gate connected P-channel metal oxide semiconductor) transistor, in which the source and the gate are short-circuited, has a decreased on-resistance.
FIG. 1 is a graph illustrating a voltage-current characteristic of a related-art high-voltage PMOS transistor and a diode, which are equal in width of a p-n junction. The graph indicates that the high voltage PMOS transistor has a larger breakdown current (and a smaller on-resistance) than those of the diode.
However, such high-voltage PMOS transistor still needs a further decrease in on-resistance in order to efficiently function as a protection element of an electrostatic discharge protection circuit of a semiconductor device.